The 3rd generation partnership project (3GPP) has given rise to a discussion about provision of many small cells within the coverage of a large cell (macrocell) in such a manner that the small cells use a different frequency band (e.g., a frequency band higher than a frequency band used by the macrocell). For instance, as shown in an example of FIG. 1, a plurality of small cells that use a 3.5-GHz frequency band are stationed within the coverage area of a macrocell that uses a 2-GHz frequency band.
With regard to LTE Release 12, a discussion has begun about a technique called Dual Connectivity whereby two types of base stations operate in coordination in a layer above a media access control (MAC) layer in the foregoing environment. The two types of base stations are a master eNB (e.g., a macro base station) that takes the initiative in communication control, and a secondary eNB (e.g., a small cell base station) that operates under the communication control. With Dual Connectivity, the two types of base stations are connected by a wired or wireless backhaul line, and the transmission traffic to a terminal is passed to the secondary eNB by way of the master eNB, for example. Thereafter, a signal that has been passed is transmitted as a wireless signal from the secondary eNB to the terminal. A signal transmitted from the terminal to the secondary eNB is transferred from the secondary eNB to the master eNB. That is to say, the entirety or a large part of the traffic passes through the master eNB at some point.
Although the backhaul line is required to have a large capacity, e.g., 1 Gbps, the requirement for the backhaul line with regard to a delay is, for example, at least 10 ms and is thus not relatively strict. Therefore, with Dual Connectivity, highly real-time data, such as control data for maintaining mobility (C-plane) and voice call traffic, is transmitted from the master eNB to the terminal, as shown in FIG. 2. On the other hand, best-effort user data that need not be real-time (U-plane), such as data for web browsing, is transmitted from the secondary eNB to the terminal. This enables data offloading to the secondary eNB without conducting a handover in association with connection to the secondary eNB.